A telephone loop normally extends from a central office to an interface connection location and then to terminal equipment on a subscriber's premises. The interface connection location which may be referred to as a reference point is usually adjacent to or on the subscriber's premises and may, for example, be in the vicinity of the well-known protector unit. One such protector unit which protects telephone loops from excessive voltages and currents is shown in U.S. Pat. No. 4,215,381 which issued on July 29, 1980 in the name of R. F. Heisinger.
When a fault occurs in the subscriber loop, the fault either exists between the central office and the reference point or between the reference point and the terminal equipment. A fault is any trouble which prevents the successful operation of the subscriber loop. If the trouble exists between the central office and the reference point, a repair can be made without entering the subscriber's premises. On the other hand, it the trouble exists on the subscriber's side of the reference point, it becomes necessary to enter the premises and to test the equipment.
If a determination as to fault location with respect to the reference point could be made, at least one maintenance problem would be overcome. Recent changes in the organization of maintenance activities within several operating companies have established a separation of craft responsibilities for local loop maintenance. Under those changes, a cable repair technician is dispatched to repair loop faults that occur between the central office and the protector unit at the subscriber's premises. On the other hand, if a fault occurs beyond the protector unit, an installer repairperson is functional. If a determination can be made as to whether a fault in the subscriber loop is located between the central office and the reference point or between the reference point and the terminal equipment, then the functionally responsible craftperson can be dispatched.
A determination as to the location of a fault with respect to the reference point is also useful from another standpoint. Recent changes in the telecommunications industry have resulted in the sale of customer premise telephone equipment by a number of suppliers. This equipment is generally purchased in retail outlets and is capable of being connected into wall terminals by means of modular plug-terminated cords. Modular plugs that terminate the cords and modular jacks which are mounted in the wall terminals are well known and are shown for example in U.S. Pat. Nos. 4,148,539 and 3,990,764 which issued on Apr. 10, 1979 and Nov. 9, 1976 in the names of E. C. Hardesty and C. L. Krumreich, respectively.
One problem that arises because of the interconnection of equipment, which may be manufactured by various concerns, into transmission circuits which are owned and operated by telephone operating companies relates to maintenance. The operating companies are not responsible for repairing equipment which is not owned or sold by them. Should a fault occur between the central office and a particular subscriber's station equipment, a determination must be made as to its location. Otherwise, the operating company would have to resort to the inefficient procedure of dispatching a craftperson to the subscriber's premises not knowing if the fault be there or outside the premises in the loop to which that equipment is connected. If such a determination could be made soon after an outage is reported, both the operating company and the subscriber would benefit. An appropriate telephone operating company craftperson could be dispatched to the premises if the fault is on the company side of a reference point, or the subscriber could be timely advised to seek repair help elsewhere if the fault is on the subscriber side of the reference point.
This latter determination is complicated by the fact that at least, for the majority of subscribers, wiring from the protector to the terminal equipment is owned by the telephone company. In order to locate a fault with respect to the point of demarcation between subscriber equipment and telephone company owned equipment, additional tests may be required or additional apparatus must be located adjacent to the terminal equipment itself. Of course, if in the future the subscriber were to install and own the inside wiring, the point of demarcation becomes the reference point, desirably on the outside of the subscriber's premises.
In one prior art method for overcoming one of the aforementioned problems, a test is made by mechanically disconnecting the loop at the subscriber's premises and by connecting the loop to a termination network of known impedance. The known impedance will be measured in response to a test signal if no fault lies between the reference point and the central office. If the fault lies between the central office and the reference point, the known impedance will not be detected. It follows that in the former case, repairs are needed between the reference point and the terminal equipment while in the latter, they are required between the central office and the reference point. While this method offers a solution to the one problem, its reliability, which depends on making connections through relay contacts, is a problem. Also, there is the necessity of maintaining records to determine whether a particular loop is equipped for such a test. See U.S. Pat. No. 4,109,112.
The prior art for the testing of subscriber loop serviceability also includes U.S. Pat. No. 4,054,759. In response to a predetermined voltage being maintained on the subscriber loop, an oscillator replaces terminal equipment in a subscriber loop through the operation of relay contacts. The characteristic of the loop current as determined by the tone detector at the central office is an indication of the serviceability of the subscriber loop.
While devices for solving at least one of the hereinbefore described problems are available in the marketplace, they do not appear to have the reliability desired for telephone communications and they are too expensive for the widespread use needed in today's industry. What is needed is a highly reliable, inexpensive device which is easily installed adjacent to a subscriber's premises and which is effective to facilitate the rapid servicing of telephone system faults.